Understanding climate requires long-term measurements of Earth’s atmosphere. Direct measurements of global temperatures have been recorded for only the past 140 years or so. To extend these records back in time, scientists have learned that certain natural processes preserve indirect evidence of past atmospheric conditions. Using data from glacial ice cores, tree rings, lake-bottom sediments, and ocean corals, they can estimate global temperatures going back thousands of years.

The graph below shows reconstructed temperature data for the Northern Hemisphere for the past 1000 years. Instead of actual temperatures, the graph shows annual temperature anomalies—differences from the average temperature for each year. The gray lines are error bars, showing the possible degree of error in each measurement.

  !   Examine the graph to interpret how the temperature has changed over the last 1000 years.

1. Why do you think the error bars become smaller in the more recent part of the graph?

Major Findings about Climate Change

Scientists have noted several significant trends in these data.

• 20th century surface temperatures were the warmest of any century in the past 1,000 years.
• The rate and duration of warming in the 20th century is greater than in any of the previous nine centuries.
• The 1990s were the warmest decade in the past 1,000 years.
• 1998 was the warmest year in the past 1,000 years.
• The 11 warmest years of the past 140 have occurred since 1983, with the warmest years being 1998, 1997, and 1995.
• Average global surface temperatures have increased 0.4 to 0.8 degrees Celsius over the past 140 years.